The application of comparator circuits is commonplace in modern electronic design. In general, a comparator circuit compares an input signal against a predetermined threshold and switches between output states depending on whether the input signal is above or below the trip threshold. Some of the desirable properties of a comparator circuit include a substantially zero temperature coefficient, hysterisis about the trip threshold, and a low standby current drain from the power supply. The substantially zero temperature coefficient of the comparator circuit ensures that the trip threshold does not vary with temperature which is especially useful in applications involving wide temperature variation. The hysterisis about the trip threshold establishes an upper threshold which the input signal must exceed before the comparator circuit transitions to the high output state and a lower threshold which the input signal must fall below before converting back to the low output state. The hysterisis provides noise immunity by preventing the output signal of the comparator circuit from toggling between high and low states if the input signal varies slightly about the trip threshold. The low standby current is useful in battery applications, for example in automotive designs, wherein it is desirable for the comparator circuit to draw substantially zero current from the power supply when the input signal is below the trip threshold thereby avoiding unnecessary drain from the battery source. In many applications such as the automotive example, another desirable property of the comparator circuit is compatibility with CMOS logic levels for interfacing to conventional control circuits such as a microprocessor. Most if not all conventional comparators fail to combine the aforedescribed features in a single operational circuit.
Hence, what is needed is an improved comparator circuit operating with a substantially zero temperature coefficient with hysterisis about the trip threshold point while drawing substantially zero current in standby mode.